Keep (add if not existing) xmlns attribute for generated SVG images (#23410)

Fix #23409 

Developers could browse & preview the local SVG images files directly.

It still has clear output.

![image](https://user-images.githubusercontent.com/2114189/224317107-f4b26c76-e36a-4e80-9eee-d8dc2e16421f.png)


![image](https://user-images.githubusercontent.com/2114189/224317527-2d4ca131-978c-4933-b071-4bae483f06e1.png)

---------

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: delvh <leon@kske.dev>

public/img/svg/octicon-file-diff.svg

@@ -1 +1 @@
-<svg viewBox="0 0 16 16" class="svg octicon-file-diff" width="16" height="16" aria-hidden="true"><path d="M1 1.75C1 .784 1.784 0 2.75 0h7.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16H2.75A1.75 1.75 0 0 1 1 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h10.5a.25.25 0 0 0 .25-.25V4.664a.25.25 0 0 0-.073-.177l-2.914-2.914a.25.25 0 0 0-.177-.073ZM8 3.25a.75.75 0 0 1 .75.75v1.5h1.5a.75.75 0 0 1 0 1.5h-1.5v1.5a.75.75 0 0 1-1.5 0V7h-1.5a.75.75 0 0 1 0-1.5h1.5V4A.75.75 0 0 1 8 3.25Zm-3 8a.75.75 0 0 1 .75-.75h4.5a.75.75 0 0 1 0 1.5h-4.5a.75.75 0 0 1-.75-.75Z"/></svg>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16" class="svg octicon-file-diff" width="16" height="16" aria-hidden="true"><path d="M1 1.75C1 .784 1.784 0 2.75 0h7.586c.464 0 .909.184 1.237.513l2.914 2.914c.329.328.513.773.513 1.237v9.586A1.75 1.75 0 0 1 13.25 16H2.75A1.75 1.75 0 0 1 1 14.25Zm1.75-.25a.25.25 0 0 0-.25.25v12.5c0 .138.112.25.25.25h10.5a.25.25 0 0 0 .25-.25V4.664a.25.25 0 0 0-.073-.177l-2.914-2.914a.25.25 0 0 0-.177-.073ZM8 3.25a.75.75 0 0 1 .75.75v1.5h1.5a.75.75 0 0 1 0 1.5h-1.5v1.5a.75.75 0 0 1-1.5 0V7h-1.5a.75.75 0 0 1 0-1.5h1.5V4A.75.75 0 0 1 8 3.25Zm-3 8a.75.75 0 0 1 .75-.75h4.5a.75.75 0 0 1 0 1.5h-4.5a.75.75 0 0 1-.75-.75Z"/></svg>